#include <cstdio>
#include <algorithm>
#include <cstring>
using namespace std;
int main(void){
    long long k,n,w;
    scanf("%lld%lld%lld",&k,&n,&w);
    printf("%lld\n",(1+w)*w/2*k>n?(1+w)*w/2*k-n:0);
    return 0;
}